1. Field of the Invention
This invention relates to an optical element obtained by integrally forming a resin layer on a surface of a glass member, and a method and apparatus for forming the same.
2. Description of the Prior Art
There are known methods wherein a lens having an aspherical shape which is difficult to obtain by processing a glass material is obtained by performing forming-processing of a thin film made of a resin material on a surface of a glass member. Lenses formed by such methods are in general termed replica lenses.
In forming a replica lens, as shown in FIG. 4, a glass member 50 processed in the shape of a spherical surface having a curvature close to the shape of the aspherical surface is mounted on a mold member 52 having a forming surface 52a in the shape of a desired aspherical surface. A replica lens 55 having the shape of the desired aspherical surface is formed by filling a liquid resin in a space 54 defined by the surface of the glass member 50 and the forming surface 52a of the mold member 52 and, thereafter, curing the resin.
Preceding technical references are, for example, U.S. Pat. Nos. 4,116,753, 4,623,496 and 4,738,516.
There is, for example, a method disclosed in U.S. Pat. No. 4,235,654 providing an optical filter using a replica technique.
When a resin material is subjected to forming processing in a mold as described above, the resin material in general contracts while being cured. That is, the resin material filling the space 54 between the forming surface 52a of the mold member 52 and the surface of the glass member 50 before being cured contracts after being cured, whereby the volume of the resin material becomes smaller than the space 54 defined between the forming surface 52a and the surface of the glass member 50.
The following phenomena occur due to the reduction of the volume of the resin material:
(a) The glass member 50 itself is bent by being drawn by the contracted resin material.
(b) Peeling occurs between the surface of the glass member 50 and the resin material.
(c) Peeling occurs between the forming surface 52a of the mold member 52 and the resin material.
If the glass material 50 itself is bent as in case (a), the shape of the formed aspherical surface merely deviates from the desired shape slightly. If peeling occurs between the glass member 50 and the resin material as in case (b), the peeling may cause a fatal defect. When peeling occurs between the forming surface 52a and the resin material as in case (c), the entire surface of the resin material does not uniformly peel, since it partially peels. Hence, a difference in the degree of contraction is produced between the peeled portion and the portion in close contact with the forming surface 52a, whereby the surface of the resin is formed in a shape having discontinuous curvatures making the line between the peeled portion and the unpeeled portion a border.
If the binding force between forming surface 52a and the resin layer is increased more than necessary in order to prevent peeling between the forming surface 52a and the resin layer, the releasing force is increased when the formed replica lens 55 is released from the mold member 52, whereby the replica lens 55 will be damaged or the resin layer will be deformed when releasing the replica lens 55.